I don't want to bash something I personally not very familiar with, especially on this podium. But I don't know what the appeal of SimpleDB-like services is. I know why Google/Amazon needed them, but those reasons don't apply to 99.9% of startups.
You don't need to buy your own RAID/SAN and powerful servers to use RDBMS, nearly every modern scalable hosting provider will be happy to rent them to you just like Amazon: get an account on Slicehost for smaller projects and move up to EngineYard or Joyent for high-end stuff: they will all let you scale up as needed, use DB of your choice.
P.S. I also think the author forgot about yet another important advantage of RDBMS: there are tons of various tools for them. Yes, he mentioned import/export/backup, but the list is much longer than that.
You don't need to buy your own RAID/SAN and powerful servers to use RDBMS, nearly every modern scalable hosting provider will be happy to rent them to you just like Amazon: get an account on Slicehost for smaller projects and move up to EngineYard or Joyent for high-end stuff: they will all let you scale up as needed, use DB of your choice.
P.S. I also think the author forgot about yet another important advantage of RDBMS: there are tons of various tools for them. Yes, he mentioned import/export/backup, but the list is much longer than that.